An exhaust gas emitted from diesel engines contains PM (particulate matter) based on carbonaceous soot and SOF (soluble organic fraction) of high-boiling-point hydrocarbons. When such exhaust gas is released into the atmosphere, it may adversely affect human beings and the environment. For this reason, a PM-capturing ceramic honeycomb filter, which may be called “honeycomb filter” in short, has been disposed in an exhaust pipe connected to a diesel engine. One example of honeycomb filters for purifying an exhaust gas by removing PM is shown in FIGS. 1(a) and 1(b). The honeycomb filter 10 comprises a ceramic honeycomb structure (simply called honeycomb structure) comprising porous cell walls 2 defining large numbers of outlet-side-sealed flow paths 3 and inlet-side-sealed flow paths 4, and an outer peripheral wall 1, and upstream-side plugs 6a and downstream-side plugs 6c alternately sealing the outlet-side-sealed flow paths 3 and the inlet-side-sealed flow paths 4 on the exhaust-gas-inlet-side end 8 and the exhaust-gas-outlet-side end 9 in a checkerboard pattern.
Honeycomb filters are required to capture particulate matter from exhaust gases at high efficiency, with low pressure loss to reduce burden on engines. However, the more particulate matter captured, the higher pressure loss occurs due to the clogging of cell wall pores. Accordingly, it is necessary to burn off the captured particulate matter to regenerate honeycomb filters. Because the honeycomb filters are thus repeatedly exposed to high temperatures to burn particulate matter, they are required to have high heat resistance and high heat shock resistance. Materials for forming porous cell walls have conventionally been cordierite (5SiO2-2Al2O3-2MgO) and silicon carbide (SiC).
Cordierite cell walls are resistant to heat shock cracking, meaning excellent heat shock resistance, because of as low a thermal expansion coefficient as about 10×10−7/° C., but the combustion of much accumulated particulate matter puts honeycomb filters at too high temperatures, making it likely that the cell walls are partially melted. Accordingly, precise control is needed to avoid excess elevation of the burning temperatures, resulting in increase in the production and running costs of exhaust-gas-cleaning apparatuses.
When cell walls are formed by silicon carbide, honeycomb structures may be cracked by heat shock stress despite excellent heat resistance, because of as large a thermal expansion coefficient as 40×10−7/° C. To reduce heat stress, divided honeycomb filter parts may be integrally bonded, but its effects are not sufficient, only suffering a high cost due to division and bonding.
Recently proposed to solve the above problems is the use of aluminum titanate (Al2TiO5) for honeycomb structures. Aluminum titanate has heat resistance exceeding 1700° C., a small thermal expansion coefficient, and excellent heat shock resistance. Aluminum titanate has a small thermal expansion coefficient because of microcracks generated in a sintering process due to the anisotropic thermal expansion coefficient of aluminum titanate crystals, while the microcracks reduce the mechanical strength of the honeycomb structures. Namely, conventional aluminum titanate suffers contradiction to meet a small thermal expansion coefficient and high strength. Also, because conventional aluminum titanate is usually decomposed in a temperature range of 800-1280° C., it cannot be used stably in this temperature range for a long period of time. To solve such problems of conventional aluminum titanate, the following technologies are disclosed.
WO 05/105704 discloses an aluminum magnesium titanate crystal structure having a thermal expansion coefficient of −6×10−6 to 6×10−6 (1/K), which is formed by a solid solution, in which part of Al atoms on at least a surface layer of an aluminum magnesium titanate crystal having a composition represented by MgxAl2(l-x)Ti(1+x)O5, wherein 0.1≦x<1, are substituted by Si atoms, the ratio of aluminum magnesium titanate remaining when kept at 1100° C. for 300 hours in the air being 50% or more, and its production method. WO 05/105704 describes that the above structure has heat resistance inherent in aluminum titanate and an extremely small thermal expansion coefficient, as well as excellent heat shock resistance, high thermal decomposability and high mechanical strength. However, higher-performance honeycomb filters are recently demanded, and the honeycomb filter described in WO 05/105704 is insufficient to meet both a low thermal expansion coefficient and high strength, and not satisfactory in pressure loss characteristics. Accordingly, the method of WO 05/105704 cannot produce a honeycomb filter having heat shock resistance, strength, high-temperature stability and pressure loss characteristics improved to practically acceptable levels.
WO 06/39255 discloses a ceramic body comprising 50-95% by mass of aluminum titanate crystal phases and 5-50% by mass of glass phases, the glass phases having a composition comprising 50-90% of SiO2, 1-25% of Al2O3, 0.5-10% of TiO2, 0.5-20% of R2O, wherein R is an element selected from the group consisting of Li, Na, K, Ru, Cs and Fr, and 0.5-20% of R′O, wherein R′ is an element selected from the group consisting of Be, Mg, Ca, Ba and Ra, and its production method, and describes that the ceramic body has excellent shock resistance and heat cycle resistance and is suitably usable at high temperatures. However, recent demand of higher-performance honeycomb filters makes the honeycomb of WO 06/39255 insufficient to meet both a low thermal expansion coefficient and high strength, and unsatisfactory in pressure loss characteristics and thermal stability at 800-1250° C. Accordingly, the method of WO 06/39255 cannot produce honeycomb filters having heat shock resistance, strength, high-temperature stability and pressure loss characteristics improved to practically acceptable levels. Though the production method is not described in detail, Examples indicate that pulverized glass of a particular composition, which is fused at 1600° C., should be used as a starting material, suffering a high production cost.
JP 5-85818 A discloses aluminum titanate ceramics having crystal phases composed of 60-85% of aluminum titanate, 10-25% of rutile, 2-10% of corundum and 2-10% of mullite, and 5% or less of glass phases, and its production method, and describes that the aluminum titanate has excellent heat cycle durability and insert-castability. However, because the aluminum titanate ceramics of JP 5-85818 A contain as much rutile as 10-20%, they have large thermal expansion coefficients and poor heat shock resistance. In addition, in view of recent demand of higher-performance honeycomb filters, its pressure loss characteristics and thermal stability at 800-1250° C. are not satisfactory. Accordingly, the method of JP 5-85818 A cannot produce honeycomb filters having heat shock resistance, strength, high-temperature stability and pressure loss characteristics improved to practically acceptable levels.
JP 60-5544 B discloses a silicate-containing aluminum titanate ceramic material having a chemical composition comprising 50-60% by weight of Al2O3, 40-45% by weight of TiO2, 2-5% by weight of kaolin and 0.1-1% by weight of magnesium silicate, and made of starting materials having particle sizes of 0.6 μm or less, and describes that it has high shock resistance and mechanical strength. However, recent demand of higher-performance honeycomb filters makes the honeycomb of JP 60-5544 B insufficient to meet both a low thermal expansion coefficient and high strength, and unsatisfactory in pressure loss characteristics and thermal stability at 800-1250° C. Accordingly, the method of JP 60-5544 B cannot produce honeycomb filters having heat shock resistance, strength, high-temperature stability and pressure loss characteristics improved to practically acceptable levels.
JP 2006-96634 A discloses a porous aluminum titanate ceramic body having porosity of 51-75%, an average pore size of 10-40 μm, and a prescribed pore size distribution, and describes that such structure can provide ceramic honeycomb filters with excellent heat resistance and heat shock resistance, low pressure loss and practically acceptable strength. However, recent demand of higher-performance honeycomb filters makes the porous ceramic body of JP 2006-96634 A unsatisfactory in thermal stability at 800-1250° C. It is also insufficient to meet both a low thermal expansion coefficient and high strength, and further improvement is needed to obtain honeycomb filters having heat shock resistance, strength, high-temperature stability and pressure loss characteristics improved to practically acceptable levels.
As described above, conventional aluminum titanate is insufficient to meet both a low thermal expansion coefficient and high strength, and further improvement is needed to obtain honeycomb filters having heat shock resistance, strength, high-temperature stability and pressure loss characteristics improved to practically acceptable levels.